Zirconacyclopentenes reactions

The first protonation occurs much more rapidly at the sp3 carbon attached to zirconium than at the sp2 carbon. Reaction at the sp2 carbon attached to the zirconium in the zirconacyclopentene is very slow with methanol. As regards the second protonation of the organozirconocene, even the sp3 carbon does not react with MeOH (Eq. 2.13) [23]. 

Cleavage of the (3,(3-carbon—carbon bonds of zirconacydopentanes and zirconacyclopen-tenes is very often observed. This cleavage reaction is useful for the preparation of various zirconacycles. As examples, various transformations of zirconacyclopentenes involving (3,(3-carbon—carbon bond cleavage are shown in Eq. 2.69 [13,48],... 

Elimination of an alkoxy group or of a halogen in the case of zirconacyclopentenes has been investigated in combination with the (3, (3-carbon—carbon bond-cleavage reaction. As shown in Eq. 2.73, an OR group or a halogen at a (3-position is eliminated and trapped by the zirconium metal center [53],... 

The most versatile synthesis involves the transmetallation of a zirconiacyclopentadiene231,232 with a tin halide under catalysis with CuCl233 (and an equivalent reaction occurs with other halides of other elements of Groups 13, 14, 15, and 16) (Equation (77)). Stannacyclopentenes can be prepared by analogous reactions of zirconacyclopentenes.233... 

The remarkable effect that CuCl has on the reaction of the zirconacyclopentadienes with R2SnCl2 or SnCU (Section 3.14.9) extends to reactions involving zirconacyclopentenes and zirconacyclopentanes, providing a route to stannacyclopentenes and stannacyclopentanes (Equations (123) and (124)).233... 

Homoallylic ethers appeared much less reactive towards the f-elimination reaction with the zirconocene-alkyne complexes or zirconacyclopentenes [21]. No -elimination products were observed in these cases. In contrast, Szymo-... 

The synthetic potential of organozirconocenes is greatly expanded by their easy transformation into other organometallics. Therefore, transmetalation-based approaches to cyclopropane synthesis have been reported. The reaction of zirconacyclopentene with phthaloyl chloride in the presence of CuCl was used for the preparation of cyclopropylenolate derivatives in moderate yields (Scheme 29) [39]. 

Takahashi et al. have also developed an alternative approach to dienyl zir-conocene by ethenylzirconation of vinyl ethers. As shown in Scheme 9, the reaction of alkynes with Cp2ZrEt2 24 gives zirconacyclopentenes. Unsaturated compounds such as vinyl ethers can easily replace the ethylene moiety of the zir-conacycle to afford potentially two regioisomers 37 and 38 [47-49] (Scheme 18). As 37 and 38 are in equilibrium, 38 undergoes a / -elimination reaction to give 39 as a unique isomer. This sequence maybe considered as a vinylzirconation reaction of alkynes. Although substituted alkenyl ethers, such as 40 and 41, did... 

Oxazirconacycloheptenes, generated in situ by the reaction of a zirconacyclopentene with an aldehyde, can be reacted with a second aldehyde in the presence of CuCl. After hydrolysis, a tetrahydrofuran derived from four different components, an alkyne, ethylene, and two different aldehydes, is obtained in good isolated yield (Scheme 76) <2004T1417>. 

Zirconacyclopentanes and zirconacyclopentenes readily react with CO at -78 °C to give cyclopentanones and cyclopentenones, respectively (Scheme 34). In the same conditions, zirconacyclopentadienes do not react. But reaction can be performed in the presence of NiCl2(PPh3)2 to afford cyclopentadienones. " ... 

However, the course of the reaction can be different in relation with the structure of the starting complexes. For five-membered fused zirconacyclopentenes, zirconacyclohe-xenes(llO) are formed as a 1 1 mixture of diastereomers (Scheme 41). No equilibrium was observed between (110) and zirconocene hydride (111). In comparison, monocychc zirconacyclopentenes and six-membered fused zirconacyclopentenes directly form the product (112), resulting of a /6-hydride transfer/Q -ehmination see a-Elimination) in high yield, presumably via (111). The CG of the fused five-membered ring could be responsible for the absence of /6-hydride transfer see Hydrides). 

Zirconacyclopentenes 701 are obtained by the reaction of alkynes with Gp2ZrEt2, as shown in Scheme 170.526 The X-ray structure of 701 (R = R2 = Ph) indicates a single bond between the (3- and / -carbons of the zirconacyclo-pentene. The reaction of these zirconacyclopentenes with nitriles proceeds via (3,(3 -carbon-carbon bond cleavage of the zirconacyclopentenes to give the azazirconacyclopentadienes 702. Alkynes also react with diethyl zirconocene and electrophiles such as GlG02Et to give stereodefined ct, /3-unsaturated esters 703.527... 

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